1. Field of the Invention
The present invention relates to an InP solar cell and more particularly to a high-efficiency solar cell having superior radiation-resistant characteristics.
2. Description of the Prior Art
Research is now moving forward on devices for large capacity communication satellite systems oriented toward INS (Information Network System) architecture. The high transmission capacities of those communication systems require large supplies of electrical power. Since recent artificial satellites have a life of at least 10 years, much longer than any of previous artificial satellite systems, there is a strong need for the solar cells powering these satellites to be of much higher efficiency and more long-lived. Solar cells are subject to various types of radiation in the environment of space where satellites are operated, so that such radiation causes lattice defects in semiconductors. These lattice defects result in output drops (radiation degradation) in the solar cells. Radiation degradation is decisive of the life of a solar cell.
Conventional solar cells for use in space are of the Si solar cell type. Conventional Si solar cells lack resistance to radiation degradation because the material Si is a semiconductor having an indirect bandgap.
Measures have been adopted to deal with factors in the Si solar cell such as optimizing the conductivity type and resistivity of its Si substrate layer and using antiradiation glass covers, which do reduce radiation damage. However, the life of those solar cells in the space environment is still on the order of only five years.
Solar cells using GaAs having a direct bandgap are being tested for space applications (viz., U.S. Pat. No. 4,156,310). Although GaAs solar cells represent an improvement over Si solar cells in radiation resistance, the life of the GaAs solar cell in space is estimated at about 10 years and thus is still inadequate. Furthermore, GaAs has a high surface recombination velocity, so that the GaAs solar cell requires a window layer for suppressing the influence of such a high surface recombination velocity. The addition of such a window layer means a more complicated device structure and fabrication process.
FIG. 1 shows examples of the relative changes in photoelectric power conversion efficiency due to 1 MeV electron irradiation of a conventional n.sup.+ -p junction Si solar cell and a heteroface GaAs solar cell.
When a solar cell is used in the radiation environment of space, consideration should be given among particle beams to the 1 MeV electrons with a large flux. The fluence of 1.times.10.sup.15 cm.sup.-2 substantially corresponds to the total radiation fluence of solar cells in geostationary satellite orbit for approximately 10 years. That is to say, if the above stated solar cells are used in the space environment for approximately 10 years, the conversion efficiency of Si solar cells will be reduced to less than half and the efficiency of GaAs solar cells will be reduced to approximately 70% of their initial conversion efficiencies. Conventional Si solar cells and GaAs solar cells are deficient in their resistance to radiation degradation.
On the other hand, the theoretical conversion efficiency for InP solar cells is approximately 23% (AM0; Air Mass zero), similar to that of GaAs cells. While conversion efficiencies of 14-15% (AM2) have been obtained for CdS/InP and indium-tin-oxide (ITO)/InP heterojunction cells, only early work yielding efficiencies of approximately 2% has been reported for InP homojunction cells.
Since the crystalline structures of the CdS and InP layers on both sides of its junction differ in these heterojunction cells, crystalline defects easily arise in the junctions. In addition, because CdS is grown at high temperatures, CdS is likely to diffuse into InP and InP is likely to diffuse into CdS so that such diffusion changes the characteristics of the junction.
According to "High-efficiency InP homojunction solar cells," by G. W. Turner, et al. on pp. 400-402 of Applied Physics Letters Vol. 37(4), Aug. 15, 1980, the fabrication of InP homojunction cells with conversion efficiencies as high as 14.8% (AM1) is reported. The InP homojunction solar cells were formed from an n.sup.+ /p/p.sup.+ structure grown by liquid phase epitaxy (LPE) on (100) oriented, p.sup.+ (Zn.about.1.times.10.sup.18 cm.sup.-3) single-crystal InP substrates. A p(Zn.about.2.times.10.sup.17 cm.sup.-3) layer about 2 .mu.m thick was grown first, followed by an n.sup.+ (Sn.about.(2-5).times.10.sup.18 cm.sup.-3) layer in the thickness range between 0.05 and 1.0 .mu.m.
However, in this case, the radiation resistance and such a high conversion efficiency as 18% of GaAs cells have not been obtained and no consideration has been made of the InP homojunction solar cell for use in space.